With the popularization of small-sized electronic appliances, ceramic substrates have extensively been used for mounting various electronic parts thereon to construct such electronic appliances. Recently, monolithic ceramic substrates have been developed in order to further increase the packaging density of electronic parts mounted thereon. Monolithic ceramic substrates are generally prepared by laminating a plurality of ceramic green sheets each having on its surface a circuit pattern made from an electroconductive paste, followed by baking. Alumina has heretofore usually been used as the ceramic material for those monolithic ceramic substrates.
However, monolithic ceramic substrates of alumina are problematic in the following points.
1. Since alumina has a high sintering temperature falling between 1500 and 1600.degree. C., laminated alumina green sheets require substantial energy for baking, resulting in an increase in the production costs of the monolithic ceramic substrate.
2. The electroconductive material for inner circuits to be formed inside the monolithic ceramic substrates of alumina is limited to a high-melting-point metal that is resistant to high-temperature baking, such as W or Mo. However, since these metals have a high specific resistivity, the circuit pattern made of the metal has a high resistance value.
3. Since the thermal expansion coefficient of alumina is larger than that of silicon chips constituting semiconductor parts to be mounted on the alumina substrates, thermal stress is imparted to the silicon chips and may cause them to crack.
4. Since alumina has a high dielectric constant of about 10, the time delay for the signal transmission through circuits inside the monolithic alumina substrates is large.
Recently, there is an increasing demand for monolithic substrates for LCR-integrated LSI capable of being driven at a high frequency of over 10 MHz. Those monolithic substrates require not only the solution of the problems noted above but also to have a small temperature-dependent variation in the dielectric capacity of the substrate material of, for example, not higher than .+-.30 ppm/.degree. C.